I found a map on the web that is a bit different than what you usually see. This one is not to scale, nor is it geographically precise, but it sets out to show the highways as either vertical, horizontal, or diagonal lines. This seemed to fit the bill.
So, I took this map, used Photoshop to draw semi-transparent overlays to denote my travels, and the result is below. Not too bad, if not as precise as other efforts I have seen. It shows that I need to do some more road trips to cover some territory I have never driven. My coverage is shown in red.
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Click the map for an enlarged view